The present invention relates generally to the investment casting art, and more specifically to the after-cast operation of removing the gates from investment castings.
As will be understood by those familiar with the investment casting art, and as explained, for example, in U.S. Pat. No. 4,109,699, the disclosure of which is hereby incorporated by reference, a pattern assembly is prepared by attaching a plurality of casting patterns around a cylindrical sprue or center tree to form a branched xe2x80x9cset-upxe2x80x9d. The patterns are connected to the center tree by gates. The patterns, which are replicas of the parts to be cast in metal, are made of wax, synthetic resin or a wax and synthetic resin composition, as are the gates. The pattern set-up is coated or invested with a suitable slurry which is allowed to harden to form a refractory mold. Thereafter, the patterns, gates and center tree are removed to form the mold cavities which can be filled with molten metal that is allowed to solidify.
U.S. Pat. No. 4,123,876 shows a typical investment cast set-up that comprises a tubular center tree and rows of castings attached to the outside of the center tree by gates. The tubular center tree, castings and gates duplicate the pattern set-up which was used to make the mold and then subsequently removed to form the mold cavities. U.S. Pat. No. 4,123,876 also shows a cut-off machine for cutting through the metal gates in order to separate the castings from the center tree.
When the castings have been removed from the center tree, it is necessary to cut off the metal stub ends of the gates that extend from the castings. A conventional practice is for an operator to grasp each individual casting and press the stub end of the gate against an abrasive wheel or belt until the stub end is flush with the casting or is within a specified tolerance band. This is a labor intensive and time consuming operation. The operation is particularly difficult when the casting has a curved outer surface which requires the casting to be manipulated by the operator in order to conform the stub end of the gate to the curvature of the casting.
An alternative, mechanized operation involves the use of an automatic machine capable of grinding a plurality of parts at one time. These machines usually require a separate fixture for holding each casting during the cut-off operation. Where a number of castings are to be ground simultaneously, a separate holding means on each fixture may be required for each different casting size and shape. The very large number of fixtures that are necessary results in high tooling costs.
It is often cost effective to make parts close to near net shape by investment casting, and then to machine selected dimensions that require closer tolerances than obtainable by casting, as well as occasional features which can be produced by machining at a lower cost than by casting. The practice of casting to near net shape requires even more tooling for after cast operations, which increases the cost even further.
An object of the present invention is to improve after cast operations generally, and to eliminate the need for excessive fixturing costs in particular.
One aspect of the invention is an improvement in the manufacture of an investment casting having a gate that is removed in an after-cast operation, the improvement comprising the steps of making the casting gate with a locator capable of locating the casting in a desired position and of being gripped to hold the casting in place, gripping the locator, and cutting through the gate near the casting while gripping the locator.
Another aspect of the invention is an improved investment casting pattern including a pattern gate attached thereto, the improvement comprising a grippable locator formed on the pattern gate at a location spaced from the casting pattern.
The locator feature comprising the invention is a generic locating formation which is made integrally with the casting gate. It is not specific to any particular casting shape or gate configuration. A small number of clamps which coact with the locator on the stub end of the casting gate can be used with a large number of different casting shapes and sizes. This is especially economical when castings are ordered intermittently, since the custom fixtures used in the past can be employed only for specific casting orders and are then stored for periods of non-use.
While the locator feature has been described as an integral part of the casting gate, it will be understood that it also is an integral part of the pattern gate, since the casting and casting gate replicate the pattern and pattern gate. An added benefit of the locator feature on the pattern gate is that it can be grasped by a robot to remove the pattern with its gate from the injection mold. The robot, while clamping onto the locator of the pattern gate, can either place the pattern on a tray or in a container or, if desired, the robot can place the end of the pattern gate into a hot melt of wax, resin, etc. and then stick the end of the pattern gate on a center tree, such as shown in the above referenced U.S. Pat. No. 4,109,699. While robots have been used in the past, they are not always cost effective, especially for small orders, because of the great of variety of end-of-arm tooling needed for the large number of different pattern shapes and sizes that the robot is required to handle. The invention makes it possible to employ a robot with a generic clamp that can process large numbers of small volume casting orders.